The present invention relates to a method for forming carbon-based protective overcoat layers having improved corrosion and wear resistance, and to improved magnetic and MO recording media including an ultra-thin, carbon-based protective overcoat layer formed according to the inventive methodology. The present invention is particularly useful in the manufacture of very high areal recording density magnetic media utilized with read/write transducers operating at very low flying heights.
A magnetic recording medium, e.g., a hard disk, typically comprises a laminate of several layers, comprising a non-magnetic substrate, such as of Alxe2x80x94Mg alloy or a glass or glass-ceramic composite material, and formed sequentially on each side thereof, a polycrystalline underlayer, typically of chromium (Cr) or Cr-baed alloy, a polycrystalline magnetic recording medium layer, e.g., of a cobalt (Co)-based alloy, a hard, abrasion-resistant, protective overcoat layer, typically containing carbon (C), and a lubricant topcoat.
In operation of the magnetic recording medium, the polycrystalline magnetic recording medium layer is locally magnetized by a write transducer, or write head, to record and store information. The write transducer creates a highly concentrated magnetic field which alternates direction based on the bits of information being stored. When-the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium layer, then the grains of the polycrystalline recording medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the polycrystalline recording medium can subsequently produce an electrical response in a read transducer, allowing the stored information to be read.
Thin film magnetic recording media are conventionally employed in disk form for use with disk drives for storing large amounts of data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads. In operation, a typical contact start/stop (CSS) method commences when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions, allowing data to be recorded on and retrieved from the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the static position, and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic sequence consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk, and stopping.
As a consequence of the above-described cyclic CSS-type operation, the surface of the disk or medium surface wears off due to the sliding contact if it has insufficient abrasion resistance or lubrication quality, resulting in breakage or damage if the medium surface wears off to a great extent, whereby operation of the disk drive for performing reading and reproducing operations becomes impossible. The protective overcoat layer is formed on the surface of the polycrystalline magnetic recording medium layer so as to protect the latter from friction and like effects due to the above-described sliding action of the magnetic head. Abrasion-resistant, carbon (C)-containing protective coatings have been utilized for this purpose, and are typically formed by sputtering of a carbon target in an argon (Ar) atmosphere. Such amorphous carbon (a-C)-containing protective overcoat layers formed by sputtering have relatively strong graphitic-type bonding, and therefore exhibit a low coefficient of friction in atmospheres containing water (H2O) vapor, which characteristic is peculiar to graphite. However, the a-C layers produced in such manner have very low hardness as compared with many ceramic materials such as are employed as slider materials of thin film heads, and thus are likely to suffer from wear due to contact therewith.
In recent years, therefore, carbon-based protective overcoat layers having diamond-like hardness properties (i.e., HV of about 1,000-5,000 kg/mm2) have been developed, and films of diamond-like carbon (DLC) having a high percentage of diamond-type C-C bonding have been utilized. Such DLC films exhibit a high degree of hardness due to their diamond-like sp3 bonding structure, and in addition, exhibit the excellent sliding properties characteristic of carbon, thus affording improved sliding resistance against sliders composed of high hardness materials. Such DLC films are generally obtained by DC or RF magnetron sputtering of a carbon target in a gas atmosphere comprising a mixture of Ar gas and a hydrocarbon gas, e.g., methane, or hydrogen gas. The thus-obtained films exhibit DLC properties when a fixed amount of hydrogen is incorporated therein. Incorporation of excessive amounts of hydrogen in the films leads to gradual softening, and thus the hydrogen content of the films must be carefully regulated.
Amorphous, hydrogenated carbon films (referred to herein as a-C:H films) obtained by sputtering of carbon targets in an Ar+H2 gas mixture exhibiting diamond-like properties have also been developed for improving the tribological performance of disk drives; however, the electrical insulating properties of such type films lead to undesirable electrical charge build-up or accumulation during hard disk operation which can result in contamination, glide noise, etc. In order to solve this problem without sacrifice or diminution of the advantageous mechanical properties of such a-C:H films, attempts (for example, as disclosed in U.S. Pat. Nos. 5,540,957; 5,837,357; 5,855,746; and 5,858,182, as well as U.S. Patent Application Publication US 2001/0031382 A1 (published Oct. 18, 2001, the entire disclosures of which are incorporated herein by reference) have been made to form bi-layer structures including a lower C:H overcoat layer and an upper, nitrogen-containing C:H overcoat layer, or to dope or otherwise incorporate nitrogen (N) atoms into the surface of a C:H protective overcoat, in order to decrease the electrical resistivity thereof and/or to provide increased bonding of the lubricant topcoat layer to the protective overcoat layer.
However, the continuous increase in areal recording density of magnetic recording media requires read/write transducers operating at a commensurately lower flying height. Therefore, it would be advantageous to reduce the thickness of the carbon-based protective overcoat layer without adverse consequences. Conventional sputtered a-C:H materials are difficult to uniformly deposit and generally do not function satisfactorily at a thickness of about 30 xc3x85 or less. Specifically, conventional sputtered a-C:H films of about 30 xc3x85 thickness fail to provide adequate protection against corrosion of the underlying magnetic layer(s), particularly Co-containing ferromagnetic layers, when under environments of high temperature and humidity, and the resulting corrosion product(s) frequently are disadvantageously transferred to the transducer heads, often leading to failure of the disk drive.
The use of alternative deposition techniques for developing thinner, harder, and more dense C:H layers having the requisite mechanical and tribological properties has been studied, such as plasma enhanced chemical vapor deposition (PECVD), ion beam deposition (IBD), and filtered cathodic arc deposition (FCAD) techniques. For example, the IBD method can be utilized for forming high carbon density, hydrogenated carbon films (referred to herein as i-C:H films) that exhibit superior tribological performance at thicknesses below about 100 A. The superior tribological performance exhibited by C:H films (including i-C:H films) formed according to these alternative techniques are attributed to their greater hardness and density vis-à-vis conventional sputtered C:H films, stemming from the use of much higher carbon (C) ion energies (i.e., xcx9c50-150 eV) during deposition by means of the alternative techniques. For example, as may be seen from the graph of FIG. 1, the present inventors have determined that the density of the C:H films increases substantially linearly with increasing C ion energy during deposition, and C:H films having desirable, very high C atom densities greater than xcx9c2.0 gm/cm3 are formed when the C ion energy exceeds about 90 eV. However, such films may be insulating as deposited and, thus, suffer from the above-described drawback of electrical charge build-up during hard disk operation associated with sputtered a-C:H films.
In order to increase the electrical conductivity and to achieve better tribological performance of the C:H protective overcoat layers formed by the aforementioned alternative deposition techniques, the present inventors investigated a bi-layer approach similar to that utilized with sputtered C:H films, wherein a layer of nitrogen-doped C:H was deposited over an undoped C:H layer formed by the alternative technique (e.g., PECVD or IBD) for improving interaction between the protective overcoat and lubricant topcoat layers, the nitrogen content of the N-doped layer being controlled by regulating the flow of N2 gas to the C:H deposition chamber. However, recent experiments by the present inventors have demonstrated that when the PECVD or IBD process is utilized for forming the nitrogen-doped layer, the nitrogen content of the resultant N-doped C:H films is very low, i.e., 5 at. % or less. Another drawback associated with this nitrogen doping methodology is that the carbon density of the N-doped C:H films is decreased relative to that of the undoped C:H films, as is evident from the graph of FIG. 2. The decreased carbon density of the N-doped C:H films disadvantageously results in degradation of the tribological performance of the overcoat layer.
Accordingly, there exists a need for an improved hard, abrasion and corrosion-resistant, high carbon density, nitrogen-doped material particularly suitable for use as ultra-thin (i.e.,  less than 30 xc3x85 thick) protective overcoat layers in high areal density magnetic recording media utilized with read/write transducers operating at extremely low flying heights, and a method for manufacturing same, which method is simple, cost-effective, and fully compatible with the productivity and throughput requirements of automated manufacturing technology.
The present invention fully addresses and solves the above-described problems attendant upon the formation of ultra-thin, abrasion and corrosion-resistant, high carbon density, N-doped protective overcoat layers suitable for use with high areal density magnetic recording media, such as are employed in hard drive applications, while maintaining full compatibility with all mechanical and electrical aspects of conventional disk drive technology. In addition, the present invention enjoys utility in the formation of ultra-thin, abrasion and corrosion-resistant protective overcoat layers required in the manufacture and use of thin film-based, ultra-high recording density magneto-optical (MO) data/information storage and retrieval media in disk form and utilizing conventional Winchester disk drive technology with laser/optical-based read/write transducers operating at flying heights on the order of a few micro-inches above the media surface.
An advantage of the present invention is an improved method of forming a layer of a hard, abrasion and corrosion resistant, nitrogen-doped, high carbon density, amorphous carbon or hydrogenated carbon (C:H) material on a surface of a substrate.
Another advantage of the present invention is an improved method of forming a protective overcoat layer on a magnetic or magneto-optical (MO) recording medium, comprising a hard, abrasion and corrosion resistant, nitrogen-doped, high carbon density, amorphous carbon or hydrogenated carbon (C:H) material.
Yet another advantage of the present invention is a magnetic or magneto-optical (MO) recording medium, comprising an improved protective overcoat layer including a novel hard, abrasion and corrosion resistant, nitrogen-doped, high carbon density, amorphous carbon or hydrogenated carbon (C:H) material.
Still another advantage of the present invention is a hard, abrasion and corrosion-resistant material useful in forming a protective overcoat layer for a magnetic or magneto-optical recording medium.
A further advantage of the present invention is a thin film layer stack including means less than about 30 xc3x85 thick for protecting the thin film layer stack against wear and corrosion while providing increased bonded lubricant ratio and decreased water contact angle.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to one aspect of the present invention, the foregoing and other advantages are obtained in part by a method of forming a layer of a hard, abrasion and corrosion resistant, nitrogen-doped, high carbon density, amorphous carbon or hydrogenated carbon (C:H) material on a surface of a substrate, which method comprises the steps of:
(a) providing a substrate having at least one surface for deposition thereon;
(b) forming, by means of a process comprising generation and deposition of carbon (C) ions having energies of at least about 90 eV, a layer of an amorphous carbon or hydrogenated carbon (C:H) material on the at least one surface of the substrate, the amorphous carbon or C:H layer having a high carbon (C) density of at least about 2.0 gm/cm3; and
(c) implanting nitrogen (N) ions in the surface of the high carbon density amorphous carbon or C:H layer to form an N-doped amorphous carbon or C:H surface layer having a high carbon density substantially equal to the high carbon density layer formed in step (b).
According to embodiments of the present invention, step (b) comprises forming the amorphous carbon or C:H layer having a high carbon density by means of a process selected from plasma-enhanced chemical vapor deposition (PECVD), ion beam deposition (IBD), and filtered cathodic arc deposition (FCAD).
In accordance with particular embodiments of the present invention, step (a) comprises providing a disk-shaped substrate including a stacked plurality of layers thereon forming a magnetic or magneto-optical (MO) recording medium; and step (b) comprises forming the amorphous carbon or C:H layer having a high carbon density on the exposed surface of an uppermost layer of the stacked plurality of layers.
According to certain embodiments of the present invention, step (b) further comprises forming the amorphous carbon or C:H layer having a high carbon density to a thickness not greater than about 30 xc3x85; step (c) comprises bombarding the surface of the amorphous carbon or C:H layer having a high carbon density formed in step (b) with nitrogen (N) ions having sufficient incident energy to form the N-doped or amorphous carbon or C:H surface layer to a depth from about 3 to about 9 xc3x85 below the surface of the high carbon density C:H layer; e.g., step (c) comprises bombarding the surface of the amorphous carbon or C:H layer having a high carbon density with nitrogen ions having incident energy in the range from about 10 to about 120 eV, as e.g., wherein step (c) comprises exposing the surface of the amorphous carbon or C:H layer having a high carbon (C) density to a plasma containing nitrogen ions for an interval up to about 1.5 sec.
According to certain embodiments of the present invention, step (a) comprises providing a disk-shaped substrate including a stacked plurality of layers thereon forming a magnetic or magneto-optical (MO) recording medium; step (b) comprises forming the amorphous carbon or C:H layer having a high carbon density to a thickness not greater than about 30 xc3x85 on the exposed surface of an uppermost layer of the stacked plurality of layers, by means of a process selected from plasma-enhanced chemical vapor deposition (PECVD), ion beam deposition (IBD), and filtered cathodic arc deposition (FCAD); and step (c) comprises bombarding the surface of the amorphous carbon or C:H layer having a high carbon density with nitrogen (N) ions having sufficient incident energy to form the N-doped amorphous carbon or C:H surface layer to a depth from about 3 to about 9 xc3x85 below the surface of the amorphous carbon or C:H layer having a high carbon density, e.g., wherein: step (c) comprises exposing the surface of the amorphous carbon or C:H layer having a high carbon density to a plasma containing nitrogen ions having incident energy in the range from about 10 to about 120 eV for an interval up to about 1.5 sec.
Another aspect of the present invention is a recording medium, comprising:
(a) a substrate;
(b) a stack of thin film layers on the substrate; and
(c) a protective overcoat layer on the surface of an-uppermost layer of the stack of thin film layers, wherein:
the protective overcoat layer comprises a hard, abrasion and corrosion resistant, high carbon density, amorphous carbon or hydrogenated carbon (C:H) material including a first, undoped sub-layer (c1) in contact with the surface of the uppermost layer of the stack of thin film layers and a second, nitrogen (N)-doped sub-layer (c2) on the first, undoped sub-layer, said first and second sub-layers having substantially equal high carbon densities of at least about 2.0 gm/cm3.
According to certain embodiments of the present invention, the protective overcoat layer (c) has a combined thickness of the first and second sub-layers (c1+c2) not greater than about 30 xc3x85; and the second, N-doped sub-layer (c2) has a thickness from about 3 to about 9 xc3x85.
In accordance with particular embodiments of the present invention, the stack of thin film layers (b) comprises a stack of layers for a magnetic or magneto-optical (MO) recording medium; the substrate (a) is disk-shaped; and the medium further comprises a lubricant topcoat layer (d) on the protective overcoat layer (c).
Yet another aspect of the present invention is a hard, abrasion and corrosion-resistant material useful in forming a protective overcoat layer for a magnetic or magneto-optical (MO) recording medium, which material comprises a region of nitrogen (N)-doped, amorphous carbon or hydrogenated carbon (C:H) having a high carbon density of at least about 2.0 gm/cm3.
According to embodiments of the present invention, the material further comprises a region of undoped, amorphous carbon or hydrogenated carbon (C:H) having a high carbon density of at least about 2.0 gm/cm3.
Particular embodiments of the present invention include a magnetic or magneto-optical (MO) recording medium comprising a stack of thin film layers and a protective overcoat layer formed of the hard, abrasion and corrosion-resistant material on an uppermost layer of the layer stack, e.g., wherein the protective overcoat layer has an overall thickness not greater than about 30 xc3x85, the region of undoped, high carbon density, amorphous carbon or hydrogenated carbon (C:H) forms a first sub-layer in contact with the uppermost layer of the layer stack, and the region of N-doped, high carbon density, amorphous carbon or hydrogenated carbon (C:H) forms an about 3 to about 9 xc3x85 thick second sub-layer on the first sub-layer.
Still another aspect of the present invention is a recording medium comprising:
(a) a substrate including thereon a stacked plurality of thin film layers; and
(b) means less than about 30 xc3x85 thick and having a high carbon atom density of at least about 2.0 gm/cm3 for protecting the stacked plurality of thin film layers against wear and corrosion while providing increased bonded lubricant ratio and decreased water contact angle.
Additional advantages and aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative.